The present invention relates to semiconductor devices and, more particularly, to the mitigation of thermally induced stress where the materials from which the semiconductor device is fabricated have differing coefficients of thermal expansion.
Numerous techniques have been developed to fabricate semiconductor packages: in general, a semiconductor die is placed upon a leadframe at a selected location with bonding wires connected between bonding pads on the die and respective ones of the connection leads on the leadframe. Thereafter, the leadframe/die assembly is subject to an encapsulating or molding step by which the leadframe/die assembly is encapsulated with a suitable molding compound to form the chip package. In general, the most common molding compounds are plastic or epoxy-like materials developed to have a thermal coefficient of expansion (CTE) that is sufficiently close to that of the die so that expansion and contraction of the encapsulated structure consequent to varying temperatures does not induce stresses sufficient to cause reliability issues, including, for example, soldered-connection failures, ball-bond or nail-bond connection failures, delamination, lifting, and/or micro-cracking on or within the body of the semiconductor package.
In general, molding compounds have been developed having coefficients of thermal expansion of appropriate value to reduce stress-induced issues to acceptable levels for most applications. However, molding compounds that are optically transmissive (for use in photo responsive devices) often have CTEs that are larger than usual, in part, because the optical transmissivity characteristic often necessitates larger than usual CTEs. For example, a clear molding compound sold under the NT-332H designation by Nitto Denko Corp. of Osaka, Japan has a CTE 8-40 times that of a conventional molding compound, such SumiKon EME-G700W sold by Sumitomo Bakelite Co. of Tokyo, Japan. As a consequence, chip structures utilizing these types of clear molding compounds are subject to thermal stress issues and have a somewhat higher than usual vulnerability to stress/strain induced problems.